1. Field of the Invention
The present invention relates generally to an apparatus and method for providing broadcasting service in a wireless packet communication system. In particular, the present invention relates to an apparatus and method for controlling power allocation to pilot tones in a broadcasting system using an Orthogonal Frequency Division Multiplexing (OFDM) transmission scheme.
2. Description of the Related Art
Conventionally, a wireless transmission scheme for providing broadcasting service such as Broadcast and Multicast Service (BCMCS) has been developed aimed at reception at a fixed terminal or reception at a low-data rate mobile terminal. Active research is now being performed on technology for allowing a subscriber to receive the broadcasting service using a small-sized terminal in a high-speed mobile environment. Broadcasting technologies such as Digital Multimedia Broadcasting (DMB) and Digital Video Broadcast Handheld (DVB-H), the typical BCMCS technologies, have been developed to allow a subscriber to receive high-quality broadcasting with a small portable terminal. In addition, research also has been conducted on the DMB and DVB-H technologies to progress the existing unidirectional broadcasting service to bidirectional broadcasting service. To this end, a plan to use the existing wire/wireless communication network as a return channel is being taken into account. However, this approach has a limitation in implementing bidirectional broadcasting because different transmission schemes are used for broadcasting and communication.
Generally, a wireless packet communication system supports communication service in which information is exchanged between a particular transmitting subscriber and a particular receiving subscriber. In the communication service, different receiving subscribers receive information through different channels. However, the wireless packet communication system suffers performance degradation due to inter-channel interference because of its low channel-to-channel isolation. In order to increase the channel-to-channel isolation, the current mobile communication system uses the cellular concept along with such multiple access schemes as Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), and Frequency Division Multiple Access (FDMA). However, even the use of these technologies cannot completely remove the interference.
BCMCS service, unlike communication service, allows a transmitting subscriber to unilaterally transmit information to a plurality of receiving subscribers. There is no interference between subscribers receiving the same information because they share the same channel. However, mobile broadcasting service suffers performance degradation due to multipath fading occurring in the high-speed mobile environment. In order to address this problem, broadcasting systems designed to support mobile reception, such as Digital Video Broadcast Terrestrial (DVB-T), DVB-H, and Digital Audio Broadcast (DAB) systems, use the OFDM transmission scheme.
The use of the OFDM transmission scheme in the broadcasting systems can prevent multipath fading from causing self interference. Particularly, in broadcasting service, different base stations transmit the same broadcasting signals via a single frequency network (SFN). Thus, the OFDM transmission scheme is advantageous in that it can prevent interference between the broadcasting signals transmitted by different base stations. Therefore, application of the OFDM transmission scheme to broadcasting service can implement an interference-free environment, contributing to maximization of transmission efficiency.
A downlink of a High-Rate Packet Data (HRPD) mobile communication system uses TDMA as a multiple access scheme and Time Division Multiplexing/Code Division Multiplexing (TDM/CDM) as a multiplexing scheme.
FIG. 1 is a diagram illustrating a slot format of a downlink in a conventional HRPD mobile communication system.
As illustrated in FIG. 1, one slot has a repeated form of half slots. Pilots 103 and 108 with an NPilot-chip length are inserted in the centers of the half slots, and are used for channel estimation of a downlink at a receiving terminal. Medium Access Control (MAC) information 102, 104, 107 and 109 with an NMAC-chip length, including uplink power control information and resource allocation information, is transmitted at both ends of the pilots 103 and 108. Actual transmission data 101, 105, 106 and 110 with an NData-chip length is transmitted before and after the MAC information 102, 104, 107 and 109. In this manner, pilots, MAC information, actual data are time-multiplexed by TDM.
The MAC and data information is multiplexed with Walsh codes by CDM, and in an HRPD downlink system, lengths of Pilot, MAC, and data blocks are set to NPilot=96 chips, NMAC=64 chips, and NData=400 chip.
FIG. 2 is a diagram illustrating a slot format provided by inserting an OFDM symbol into a data transmission interval of an HRPD downlink slot for BCMCS service.
For HRPD downlink compatibility, positions and sizes of Pilot and MAC signals shown in FIG. 2 are matched to positions and sizes of Pilot and MAC signals shown in FIG. 1. That is, Pilots 103 and 108 with an NPilot-chip length are located in the centers of half slots, and MAC signals 102, 104, 107 and 109 with an NMAC-chip length are located at both sides of the Pilot signals 103 and 108. Therefore, even the existing HRPD terminal not supporting OFDM-based broadcasting service can estimate channels through pilots and receive MAC signals. OFDM symbols 121, 122, 123 and 124 are inserted into the remaining fields of the slot, that is, data transmission intervals 101, 105, 106 and 110. The OFDM symbols 121, 122, 123 and 124 are given by modulating BCMCS information.
In the existing HRPD downlink system in which NData=400 chips, a size of the OFDM symbols is set to NData=400 chips. The OFDM scheme arranges a Cyclic Prefix (CP) at the head of each of the OFDM symbols in order to prevent a reception signal time-delayed through multiple paths from causing self interference. That is, one OFDM symbol includes a CP 125 and OFDM data 126 generated by performing Inverse Fast Fourier Transform (IFFT) on BCMCS information. A size of the CP 125 is NCP chips, and for the CP 125, an NCP-chip signal is copied at the rear of the OFDM data 126 and then arranged at the head of the OFDM data 126. Therefore, a size of the OFDM data 126 becomes (NData−NCP) chips. Herein, NCP is determined depending on an allowed time delay that causes self interference. An increase in the NCP increases the number of delayed reception signals being demodulated without interference but decreases the amount of transmittable information because of a reduction in size of OFDM data. However, a decrease in the NCP increases the amount of transmittable information but reduces reception quality because of a high interference probability in a severe multipath fading environment.
In an SFN, it is common to set a size of the CP to a large value, because the same signals transmitted by several transmitters are received at a terminal at different times. In the HRPD downlink system that transmits OFDM signals for BCMCS service, it is preferable to set NCP to 80 chips (NCP=80 chips). In this case, a size of the OFDM data becomes 320 chips. This means that it is possible to perform IFFT on 320 modulation symbols and transmit the IFFT-processed symbols in an OFDM data transmission interval. Therefore, a total of 320 tones can be acquired through the OFDM scheme.
However, not all of the 320 tones can be used for data symbol transmission. Some tones located at the boundaries of a frequency band used should be used as Guard tones for preventing out-band signals from serving as interference. Because the Pilots 103 and 108 used in the existing HRPD downlink are spread with different codes at different transmitters before being transmitted, they are not appropriate to be used for channel estimation for BCMCS service provided in the SFN. Therefore, a dedicated pilot for channel estimation for OFDM signals is additionally required. A signal predefined between a transmitter and a receiver can be transmitted using a part of a tone and then used for channel estimation, and such a tone is called an OFDM-dedicated pilot tone. The OFDM scheme used in the SFN permits a relatively long time delay, resulting in severe frequency-selective fading. Accordingly, there is a need to secure pilot tones sufficient to perform channel estimation even in the severe frequency-selective fading.
FIG. 3 is a diagram illustrating a conventional tone arrangement method in an HRPD system.
Referring to FIG. 3, guard tones 201 are arranged at the boundaries of a band. Of 16 guard tones, 8 guard tones are arranged at a low frequency part of the band and the remaining 8 guard tones are arranged at a high frequency part of the band. No signal is transmitted through the guard tones, so no power is applied to the guard tones. Data tones 203 are arranged in the center of the band. Finally, pilot tones 202 are arranged at regular intervals every five tones because they are used for channel estimation. The tones are arranged in such a manner that four guard tones are followed by a pilot tone arranged at the lowest frequency and then a pilot tone is inserted again.
Similarly, a pilot tone 202 is inserted even in the field where the data tones 203 are arranged, and then four data tones 203 are followed by the pilot tone 202 and a new pilot tone 202 is arranged following the four data tones 203. In this manner, the pilot tones 202 are arranged at a frequency corresponding to a direct current (DC) component. Because the pilot tones 202 are DC tones, they are allocated no power or lower power before being transmitted.
The pilot tones 202 and the data tones 203 are different from each other in terms of the power allocated thereto. An optimal solution for a ratio of power allocated to the pilot tones 202 to power allocated to the data tones 203 should be predefined by a transmitter and a receiver because it differs according to channel conditions.
FIG. 4 is a block diagram illustrating a structure of a conventional transmitter in an HRPD system.
Referring to FIG. 4, a transmitter includes a channel encoder 301 for channel-encoding received packet data, a channel interleaver 302 for interleaving the coded packet data, a modulator 303 for modulating the interleaved packet data, a guard tone inserter 304 for inserting guard tones, and a pilot tone inserter 305 for inserting pilot tones. Further, the transmitter includes a tone power allocator 306, a Quadrature Phase Shift Keying (QPSK) spreader 307, an IFFT unit 308, a CP inserter 309, and an HRPD compatible processor 310.
Physical layer packet data generated in an upper layer is input to the channel encoder 301. The channel encoder 301 channel-encodes the packet data into a channel-coded bit stream, and outputs the channel-coded bit stream to the channel interleaver 302. The channel interleaver 302 interleaves (or performs column permutation on) the channel-coded bit stream to achieve diversity gain, and outputs the interleaved bit stream to the modulator 303. The modulator 303 modulates the interleaved bit stream into a modulation signal. The modulation signal is arranged in data tones 203.
The guard tone inserter 304 arranges the signal output from the modulator 303 in guard tones 201 located in the boundaries of a band, and the pilot tone inserter 305 arranges pilot tones 202 in the signal output from the guard tone inserter 304 at regular intervals. Thereafter, the tone power allocator 306 allocates power according to a ratio R of power allocated to pilot tones to power allocated to data tones. The transmission signal, after being allocated to all tones, is subject to QPSK spreading in the QPSK spreader 307. In the QPSK spreading process, base station signals for transmitting different BCMCS contents are multiplied by different complex Pseudo-random Noise (PN) sequences. The complex PN sequence refers to a complex sequence in which both a real component and an imaginary component include PN codes.
Because an unwanted base station signal may serve as a noise component at a receiver, the receiver can perform channel estimation separately on the channel from the unwanted base station. The complex PN sequence used in the QPSK spreading process is generated according to an input BCMCS contents identifier.
The IFFT unit 308 arranges the QPSK-spread modulation signals in positions of wanted frequency tones through an IFFT process. Thereafter, the CP inserter 309 inserts a CP into the signal output from the IFFT unit 308 so as to prevent self interference due to multipath fading, completing generation of an OFDM transmission signal. Thereafter, the HRPD compatible processor 310 follows an HRPD transmission process to insert Pilots 103 and 108, and MAC information 102, 104, 107 and 109. The finally transmitted signal has a slot format shown in FIG. 2.
With reference to FIGS. 5A and 5B, a description will now be made of a format for transmitting the OFDM BCMCS slot between CDM slots. FIG. 5A is a diagram illustrating a format for transmitting an OFDM BCMCS slot between CDM slots. Herein, the CDM slot has the slot format shown in FIG. 1, and includes a CDM-multiplexed signal in its data field. The OFDM BCMCS slot has the slot format shown in FIG. 2.
With reference to FIG. 5A, a description will now be made of a channel estimation process for each OFDM symbol at a terminal upon receiving an OFDM BCMCS slot 402 transmitted between CDM slots 401 and 403.
The OFDM BCMCS slot 402 includes therein four OFDM symbols 121, 122, 123 and 124. Reference numerals 121 and 124 indicate OFDM symbols located in the boundaries of the slot, and reference numerals 122 and 123 indicate OFDM symbols located in the center of the slot.
Generally, because a length of OFDM symbols is determined such that channels are not subject to change in the OFDM symbols, a channel change between adjacent OFDM symbols may not be significant. Therefore, the OFDM symbols located in the slot center can use pilot tones of the boundary OFDM symbols in order to estimate the channels. For example, not only the pilot tones of the OFDM symbol 122 but also the pilot tones of the OFDM symbols 121 and 123 are used to estimate channels of the OFDM symbol 122, thereby improving channel estimation performance.
However, the OFDM symbols located in the slot boundaries have a limitation in using pilot tones of adjacent OFDM symbols in the channel estimation process. More specifically, pilot tones used to estimate channels of the OFDM symbol 121 include pilot tones of the OFDM symbol 121 and pilot tones of the OFDM symbol 122. This is because there is no pilot tone to be used for channel estimation because a CDM slot other than the BCMCS slot was transmitted before transmission of the OFDM symbol 121. Therefore, the OFDM symbols 122 and 123 located in the center of the OFDM BCMCS slot are superior to the OFDM symbols 121 and 124 located in the slot boundaries in terms of the channel estimation performance. This is because the same value is used for a ratio R of power allocated to the individual pilot tones to power allocated to the individual data tones regardless of the positions of the OFDM symbols.
As a result, compared with the OFDM symbols located in the center of the OFDM BCMCS slot, the OFDM symbols located in the slot boundaries have higher reception error probability occurring during data transmission.
This phenomenon occurs even when OFDM BCMCS slots are continuously transmitted as shown in FIG. 5B. Reference numerals 405, 406 and 407 all represent OFDM BCMCS slots that transmit different broadcasting information. A terminal receiving broadcasting information of the OFDM BCMCS slot 406 is not required to receive the OFDM BCMCS slots 405 and 407. Therefore, even in the situation where OFDM BCMCS slots are continuously transmitted, the reception error probability can differ according to positions of the OFDM symbols.